Nowadays, nitric oxide, NO, has become known as a medication against pulmonary hypertension, in other words, as a vasodilator. The advantage of treatment with the widely described NO mixtures is that NO only acts locally, that is to say, in the pulmonary circulation system, and not systemically. The NO gas mixture is administered to patients through inhalation. This means that other gas components, especially air, oxygen or other gases containing oxygen, are admixed to the therapeutically effective gas by means of various techniques.
For ventilated patients, European patent application EP 621 051 A2 discloses the procedure of metering in the nitrogen oxide mixture so that it is proportional to the volumetric flow rate, in other words, as a function of the inspiratory, inhaled flow of the air-oxygen mixture. The flow of the NO mixture can also be metered in so that it corresponds to the inhaled volume.
U.S. Pat. No. 5,839,433 or world patent WO 98/31282 discloses additional techniques for the administration of NO to a patient. These publications describe the so-called spike, peak or pulse techniques. For this purpose, a valve is usually opened for a certain period of time and then closed again during the inspiratory phase in order to dispense a certain volume of NO to the lungs of the patient. This technique is normally—although not exclusively—employed for spontaneously breathing patients, that is to say, for patients who are not hooked up to a ventilator.
Irrespective of which method is chosen for the dosing of NO, there is always a negative side effect associated with NO treatment, namely, the fact that NO is converted into NO2 over the course of time when NO is mixed together with a gas that contains oxygen. Since air consists of approximately 21% oxygen, this phenomenon also occurs in air. If more time passes during which NO is present in the air or in oxygen, then more NO2 is formed. For instance, more NO2 is generated when the NO therapy is started, interrupted or resumed—since there is more time for oxidation. In other words, this is the case whenever time is available for further oxidation and whenever air and NO are present in the feed tubes and in the areas between the individual devices or else in the patient connector, which can be, for instance, a nosepiece or a mask. During such pauses, there is usually still NO in the lines and it can then react to form NO2. Then, as a rule, the gas present in the system contains a relatively large amount of NO2 which either has to be purged or else it reaches the patient. Since harmful effects already occur at minute concentrations, even the smallest amounts of NO2 should not be tolerated. This problem occurs particularly often—although not exclusively—in the case of spontaneously breathing patients whereby, for example, nosepieces or masks that do not fit well fail to trigger the desired action, namely, the dosing of NO mixtures. As a result, the mixture remains in the feed line and then reacts in the presence of oxygen to form NO2. But also due to some other erroneous detection by the sensor which triggers the start of the inspiration and thus the NO pulse to the patient, or else due to other events that prevent the initiation of the next gas mixture pulse or pulses, the NO mixture present in the feed line together with air/oxygen can lead to an elevated formation of NO2. As a result, after a renewed triggering, the patient receives gas that has an elevated content of NO2.
In order to eliminate this drawback, U.S. Pat. Nos. 6,125,846 and U.S. Pat. No. 6,109,260 each disclose a device and a method comprising a purging process. This purging process is done in such a way that, whenever there is a prolonged pause in the administration or else an apnea pause, the NO2 mixture is purged by a prolonged continuous volumetric flow of the treatment gas, namely, NO, in order to purge the NO2 mixture. A disadvantage of this approach is that the purging gas itself contains NO so that it can once again react to form NO2 during the next pause. In the worse-case scenario, the patient then once again inhales NO2.